fix(dashboard): improve i18n UX — full-word lang toggle + auto-refresh panels + error.* namespace

Language toggle button now shows "中文"/"English" full words instead of "中"/"EN" abbreviations, with dynamic title tooltip ("Switch to English" / "切换到中文"). toggleLang() made async and triggers refreshActivePanelI18n() to reload the active panel's content after locale switch, so users see translated UI immediately without manual navigation.